This invention relates generally to determining if a telephone loop is capable of carrying Digital Subscriber Line (DSL) loop signals.
Deployment of broadband services on a telephone loop is severely limited by the inherent properties of the copper cable and, in part, because initial deployment of the copper cable was aimed primarily at providing voice services to subscribers. Until the telephone loop cable and plant are upgraded or replaced, as by installation of optical fiber loops, advanced digital signal processing holds great promise today for subscribers who desire broadband services such as high speed internet access, remote Local Area Network (LAN) access and switched digital video. Technological advances have brought about DSL technology at high data rates, e.g., High-rate DSL (HDSL), Asymmetric DSL (ADSL) and Very high-speed DSL (VDSL). For example, using ADSL technology, information signals are modulated by ADSL modems onto copper telephone loops at passband frequencies so that Plain Old Telephone Service (POTS) or another baseband service may be carried on the same pair of copper wires. Using the existing copper telephone loop is extremely cost effective as the installation of new cable and structure along with their associated labor and material costs are avoided.
Deployment of DSL technology is limited by the transmission characteristics of the telephone loop. The transmission characteristics of the telephone loop depend on the length of the copper line, its gauge, the presence of bridged taps, the quality of splitters, the integrity of the shielding, loading coils, impedance mismatches and other loop impairments. Specifically, line loss increases with line length and attenuation increases with increasing frequency and decreases as wire diameter increases. If the telephone loop is too long, then transmission of DSL signals at high data rates from the CO to the subscriber's termination location and vice-versa may not be possible. There are particular points along the telephone loop between the subscriber's termination location and the originating CO where the loop is particularly susceptible to ingress interference. These points include, for example, the location of a bridged tap, the drop wire from the telephone pole to the home, and the wires within the home. At the aforementioned points ingress noise may be coupled into the loop. The presence of other telephone terminals connected to other pairs in the cable also introduces noise into the DSL signal. Furthermore, bridged taps create more loss, distortion, and echo. All these factors serve to limit the data transfer rate at which a subscriber may be connected to a broadband service provider over the telephone loop and are a major cause of connection problems subscribers currently face in making data connections via the public switched telephone network.
Service providers have several options to determine the environment the DSL signal operates in before they commit to service when a subscriber requests DSL service. The service provider may query the outside plant records to determine the loop configuration. Outside plant records more than likely would have been constructed from the original design records. In many cases, the records available are outdated and do not reflect changes that may have occurred in the outside plant as a result of maintenance and service orders. The end result is that the records are usually inaccurate and may not be relied upon to provide information required by the carrier to predict a telephone loop's ability to support DSL service. The approach described above does not provide the telephone loop characteristic information with a degree of accuracy required to confidently predict DSL performance over the loop. As such, before a particular subscriber may utilize DSL technology for his or her broadband services, the broadband service provider has to determine or have determined the viability of deploying DSL to that subscriber. Thus, there is a need for a system and technique that is able to determine whether the telephone loop is capable of carrying a DSL signal at either the subscriber's premises without any intervention or cooperation from the telephone company or Internet Service Provider (ISP) or at the central office (CO) without cooperation from subscribers.